Chapter 48: Nuerons

Overview: Command and Control Center

• The human brain contains about 100 billion nerve cells, or neurons

• Each neuron may communicate with thousands of other neurons

• Functional magnetic resonance imaging is a technology that can reconstruct a three-dimensional map of brain activity

• Brain imaging and other methods reveal that groups of neurons function in specialized circuits dedicated to different tasks

Brainbow mouse section of brainhttp://www.conncad.com/gallery/brainbow.html

Concept 48.1: Nervous systems consist of circuits of neurons and supporting cells

• All animals except sponges have a nervous system

• What distinguishes nervous systems of different animal groups is how neurons are organized into circuits

Nerve net

Hydra (cnidarian)

Radialnerve

Nervering

Sea star (echinoderm)

The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets

Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring

Ganglia

Squid (mollusc)

Salamander (chordate)

Spinalcord(dorsalnervecord)

Sensoryganglion

•Nervous systems in molluscs correlate with lifestyles

•Sessile molluscs have simple systems, whereas more complex molluscs have more sophisticated systems

LE 48-3

SensorSensory input

Motor output

Integration

Effector Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

In vertebrates, the central nervous system consists of a brain and dorsal spinal cord

The PNS connects to the CNSNervous systems process information in three stages: sensory input, integration, and motor output

• Sensory neurons transmit information from sensors that detect external stimuli and internal conditions

• Sensory information is sent to the CNS, where interneurons integrate the information

• Motor output leaves the CNS via motor neurons, which communicate with effector cells

• The three stages of information processing are illustrated in the knee-jerk reflex

LE 48-4

Quadricepsmuscle

Cell body ofsensory neuron in dorsal rootganglion

Sensory neuron

Spinal cord(cross section)

Whitematter

Hamstringmuscle

Graymatter

Motor neuronInterneuron

Neuron Structure

• Most of a neuron’s organelles are in the cell body

• Most neurons have dendrites, highly branched extensions that receive signals from other neurons

• The axon is typically a much longer extension that transmits signals to other cells at synapses

• Many axons are covered with a myelin sheath

LE 48-5

Dendrites

Cell body

Nucleus

Axon hillock Axon

Signaldirection

Presynaptic cellMyelin sheath

Synapticterminals

Synapse

Postsynaptic cell

Dendrites

Cell body

InterneuronsSensory neuron Motor neuron

Neurons have a wide variety of shapes that reflect input and output interactions

Supporting Cells (Glia)

• Glia are essential for structural integrity of the nervous system and for functioning of neurons

• Types of glia: astrocytes, radial glia, oligodendrocytes, and Schwann cells

In the CNS, astrocytes provide structural support for neurons and regulate extracellular concentrations of ions and neurotransmitters

LE 48-8

Axon Nodes ofRanvier

Schwanncell

Myelin sheathNucleus ofSchwann cell

Schwanncell

Nodes of Ranvier

Layers of myelinAxon

0.1 µm

Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) form the myelin sheaths around axons of many vertebrate neurons

Myelination in the central and peripheral nervous systems

Dorsal view of the human brain showing the progression of myelination (“white matter”) over the cortical surface during adolescence

Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron

• Across its plasma membrane, every cell has a voltage called a membrane potential

• The cell’s inside is negative relative to the outside

• Membrane potential of a cell can be measured

• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) form the myelin sheaths around axons of many vertebrate neurons

Microelectrode

Referenceelectrode

Voltagerecorder

–70 mV

The Resting Potential

• Resting potential is the membrane potential of a neuron that is not transmitting signals

• Resting potential depends on ionic gradients across the plasma membrane

• Concentration of Na+ is higher in the extracellular fluid than in the cytosol

• The opposite is true for K+

• By modeling a neuron with an artificial membrane, we can better understand resting potential

LE 48-10

CYTOSOL EXTRACELLULARFLUID

[Na+]15 mM

[K+]150 mM

[A–]100 mM

[Na+]150 mM

[K+]5 mM

[Cl–]120 mM[Cl–]

Plasmamembrane

LE 48-11

150 mMKCl

Innerchamber

Outerchamber

–92 mV

Potassiumchannel

Membrane selectively permeable to K+ Membrane selectively permeable to Na+

5 mMKCl

Artificialmembrane

150 mMNaCl

Innerchamber

Outerchamber

+62 mV

Sodiumchannel

15 mMNaCl

• A neuron that is not transmitting signals contains many open K+ channels and fewer open Na+ channels in its plasma membrane

• Diffusion of K+ and Na+ leads to a separation of charges across the membrane, producing the resting potential

Gated Ion Channels

• Gated ion channels open or close in response to one of three stimuli:

– Stretch-gated ion channels open when the membrane is mechanically deformed

– Ligand-gated ion channels open or close when a specific chemical binds to the channel

– Voltage-gated ion channels respond to a change in membrane potential

Hyperpolarizations

Graded potential hyperpolarizations Graded potential depolarizations

5Time (msec)

Restingpotential

Threshold

–100

Stimuli+50

Depolarizations

5Time (msec)

Restingpotential

Threshold

–100

Stimuli+50

Action potential

5Time (msec)

Restingpotential

Threshold

–100

Stronger depolarizing stimulus

+50Actionpotential

If a cell has gated ion channels, its membrane potential may change in response to stimuli that open or close those channels

Some stimuli trigger a hyperpolarization, an increase in magnitude of the membrane potentialOther stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential

• Hyperpolarization and depolarization are called graded potentials

• The magnitude of the change in membrane potential varies with the strength of the stimulus

Production of Action Potentials

• Depolarizations are usually graded only up to a certain membrane voltage, called the threshold

• A stimulus strong enough to produce depolarization that reaches the threshold triggers a response called an action potential

• An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane

• It carries information along axons

• Voltage-gated Na+ and K+ channels are involved in producing an action potential

• When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell

• As the action potential subsides, K+ channels open, and K+ flows out of the cell

• During the refractory period after an action potential, a second action potential cannot be initiated

Action Potential

Resting potential

Threshold

Actionpotential

Time–100

Potassiumchannel

Extracellular fluid

Plasma membrane

Resting state

Inactivationgate

Activationgates

Sodiumchannel K+

Cytosol

Depolarization

Rising phase of the action potential

Falling phase of the action potential

Undershoot

An action potential is generated as Na+ flows inward across the membrane at one location.

Action potential

Action potentialK+

The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

Action potentialK+

The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

Conduction of Action Potentials

•An action potential can travel long distances by regenerating itself along the axon

•At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane

Conduction Speed

• The speed of an action potential increases with the axon’s diameter

• In vertebrates, axons are myelinated, also causing an action potential’s speed to increase

• Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

LE 48-15

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Myelinsheath

Na+ and K+ channels are concentrated at the nodes to initiate a series of action potentials

Concept 48.4: Neurons communicate with other cells at synapses

• In an electrical synapse, current flows directly from one cell to another via a gap junction

• The vast majority of synapses are chemical synapses

• In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters stored in the synaptic terminal

LE 48-16

Postsynapticneuron

Synapticterminalsof pre-synapticneurons

LE 48-17

Postsynaptic cellPresynaptic cell

Synaptic vesiclescontainingneurotransmitter

Presynaptic membrane

Voltage-gatedCa2+ channel

Ca2+Postsynaptic membrane

Postsynaptic membrane

Neuro-transmitter

Ligand-gatedion channel

Ligand-gatedion channels

Synaptic cleft

When an action potential reaches a terminal, the final result is release of neurotransmitters into the synaptic cleft

•Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels

• Postsynaptic potentials fall into two categories:

– Excitatory postsynaptic potentials (EPSPs)

– Inhibitory postsynaptic potentials (IPSPs)

• After release, the neurotransmitter diffuses out of the synaptic cleft

• It may be taken up by surrounding cells and degraded by enzymes

Summation of Postsynaptic Potentials

• Unlike action potentials, postsynaptic potentials are graded and do not regenerate

• Most neurons have many synapses on their dendrites and cell body

• A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron

Postsynaptic neuron

Terminal branchof presynaptic neuron

Axonhillock

Actionpotential

E1E1 + E2

Spatial summationof EPSP and IPSP

Spatial summation

I E1 + I

Actionpotential

Temporal summation

Threshold of axon ofpostsynaptic neuron

Subthreshold, nosummation

Restingpotential

•If two EPSPs are produced in rapid succession, an effect called temporal summation occurs

•In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

•Through summation, an IPSP can counter the effect of an EPSP

Indirect Synaptic Transmission

• In indirect synaptic transmission, a neurotransmitter binds to a receptor that is not part of an ion channel

• This binding activates a signal transduction pathway involving a second messenger in the postsynaptic cell

• Effects of indirect synaptic transmission have a slower onset but last longer

Neurotransmitters• The same neurotransmitter can produce different effects in

different types of cells

Acetylcholine

• Acetylcholine is a common neurotransmitter in vertebrates and invertebrates

• It can be inhibitory or excitatory

Biogenic Amines

• Biogenic amines include epinephrine, norepinephrine, dopamine, and serotonin

• They are active in the CNS and PNS

Amino Acids and Peptides

• Four amino acids are known to function as neurotransmitters in the CNS

• Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters

• Gases such as nitric oxide and carbon monoxide are local regulators in the PNS

Chapter 48: Nuerons

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